Li2O-Al2O3-SiO2-BASED CRYSTALLIZED GLASS

ABSTRACT

Provided is a Li 2 O—Al 2 O 3 —SiO 2 -based crystallized glass, comprising, as a composition in terms of mass %, 55 to 75% of SiO 2 , 20.5 to 27% of Al 2 O 3 , 2% or more of Li 2 O, 1.5 to 3% of TiO 2 , 3.8 to 5% of TiO 2 +ZrO 2 , and 0.1 to 0.5% of SnO 2 , and satisfying the relationships of 3.7≦Li 2 O+0.741MgO+0.367ZnO≦4.5 and SrO+1.847CaO≦0.5.

TECHNICAL FIELD

The present invention relates to an Li₂O—Al₂O₃—SiO₂-based crystallizedglass, and more specifically, to an Li₂O—Al₂O₃—SiO₂-based crystallizedglass suitable for heat resistant applications such as a front windowfor a kerosene stove, a wood stove, and the like.

BACKGROUND ART

An Li₂O—Al₂O₃—SiO₂-based crystallized glass has been conventionally usedas a material for a front window for a kerosene stove, a wood stove, orthe like, a substrate for a high-tech product such as a substrate for acolor filter or an image sensor, a setter for baking an electronic part,a tray for a microwave oven, a top plate for induction heating cooking,a window glass for a fire prevention door, or the like. For example,Patent Literatures 1 to 3 each disclose an Li₂O—Al₂O₃—SiO₂-basedcrystallized glass comprising an Li₂O—Al₂O₃—SiO₂-based crystal, such asa β-quartz solid solution (Li₂O.Al₂O₃.nSiO₂ (provided that 4>n≧2)) or aβ-spodumene solid solution (Li₂O.Al₂O₃.nSiO₂ (provided that n≧4)),precipitated therein as a main crystal.

The Li₂O—Al₂O₃—SiO₂-based crystallized glass has a low thermal expansioncoefficient and a high mechanical strength, and hence has an excellentthermal property. Further, appropriate adjustment of conditions of heattreatment in a crystallization step allows the kind of crystalprecipitated in the Li₂O—Al₂O₃—SiO₂-based crystallized glass to becontrolled, and hence a transparent crystallized glass (a β-quartz solidsolution precipitates) can be easily manufactured.

When the crystallized glass of this kind is produced, a glass batchneeds to be melted at high temperature exceeding 1400° C. Thus, used asa fining agent added in the glass batch is As₂O₃ or Sb₂O₃, which iscapable of generating a fining gas in a large amount during the meltingat high temperature. However, As₂O₃ and Sb₂O₃ are highly toxic and maypollute an environment, for example, in a production process of theglass or at the time of treating waste glass.

Thus, SnO₂ and Cl have been proposed as substitute fining agents forAs₂O₃ and Sb₂O₃ (see, for example, Patent Literatures 4 and 5). However,Cl is liable to erode a metal mold or a metal roll during glassformation, with the result that the surface quality of the glass maybedegraded. From the viewpoint of preventing such the problem, SnO₂ ispreferably used as a fining agent.

As described in Patent Literatures 4 and 5, SnO₂ has a function ofheightening coloring caused by Fe₂O₃, TiO₂, or the like, and henceinvolves a problem in that a yellow tone of a transparent crystallizedglass prevails, which is not preferred in terms of the outer appearancethereof. Thus, when SnO₂ is used, it is preferred that the content ofFe₂O₃ contaminated as an impurity component be reduced and that thecontent of TiO₂ in a glass batch be also reduced. However, TiO₂ is acomponent of a crystal nucleus, and hence, when the content of TiO₂ isreduced, an optimum firing temperature range becomes narrower, with theresult that the generation amount of crystal nuclei is liable to besmaller. When crystallization progresses in the presence of a smallamount of crystal nuclei, a large amount of coarse crystals aregenerated, causing a problem in that the crystallized glass is liable tobecome cloudy, thereby losing transparency.

As another method of suppressing the coloring of a transparentcrystallized glass, there is known a method involving adding a coloranthaving a relationship of a complementary color with the coloring causedby Fe₂O₃, TiO₂, or the like, thereby fading the coloring. It has beenconventionally known that Nd₂O₃ is particularly effective for fadingcolors of the Li₂O—Al₂O₃—SiO₂-based crystallized glass (see, forexample, Patent Literature 6). Thus, even when the yellow color tone ofthe Li₂O—Al₂O₃—SiO₂-based crystallized glass becomes prevailed by theaddition of SnO₂, such the yellow color can be faded by adding Nd₂O₃.

Note that it is possible to obtain a white opaque crystallized glass inwhich a β-spodumene solid solution is precipitated, by carrying outcrystallization under a proper heat treatment condition in a productionprocess of the Li₂O—Al₂O₃—SiO₂-based crystallized glass.

CITATION LIST

Patent Literature 1: JP 39-21049 B

Patent Literature 2: JP 40-20182 B

Patent Literature 3: JP 01-308845 A

Patent Literature 4: JP 11-228180 A

Patent Literature 5: JP 11-228181 A

Patent Literature 6: U.S. Pat. No. 4,093,468 B

SUMMARY OF INVENTION Technical Problem

The color fading of a transparent crystallized glass caused by Nd₂O₃ is,so to speak, a technique involving converting yellow coloring to anachromatic color by superimposing blue coloring caused by Nd₂O₃ over theyellow coloring, which involves such a problem in that: thetransmittance in a visible region deteriorates so that the outerappearance of the transparent crystallized glass looks dark, and hencethe transparency thereof is liable to be impaired.

In addition, there has also been a problem in that a white crystallizedglass in which a β-spodumene solid solution is precipitated is liable tohave a higher thermal expansion coefficient and a higher dielectricloss. In particular, if the crystallized glass has a higher dielectricloss, when the crystallized glass is used for applications in whichelectromagnetic waves are used, such as a tray for a microwave oven, thetemperature thereof locally rises, causing the breakage thereof.

Therefore, an object of the present invention is to provide aLi₂O—Al₂O₃—SiO₂-based crystallized glass, in which SnO₂ is used as asubstitute fining agent for As₂O₃ and Sb₂O₃, having a reduced yellowcoloring caused by Fe₂O₃, TiO₂, or the like and an excellenttransparency.

Another object of the present invention is to provide a whiteLi₂—O—Al₂O₃—SiO₂-based crystallized glass in which a low thermalexpansion property and a low dielectric loss property can be achieved.

Solution to Problem

The inventors of the present invention have made studies into amechanism through which the coloring caused by components such as TiO₂and Fe₂O₃ is prevailed by SnO₂. As a result, the inventors have foundthat the above-mentioned problems can be solved by limiting the ratio ofeach component in a crystallized glass to a particular range, and thefinding is proposed as the present invention.

That is, the present invention provides a Li₂O—Al₂O—SiO₂-basedcrystallized glass, comprising, as a composition in terms of mass %, 55to 75% of SiO₂, 20.5 to 27% of Al₂O₃, 2% or more of Li₂O, 1.5 to 3% ofTiO₂, 3.8 to 5% of TiO₂+ZrO₂, and 0.1 to 0.5% of SnO₂, and satisfyingthe relationships of 3.7≦Li₂O+0.741MgO+0.367ZnO≦4.5 andSrO+1.847CaO≦0.5.

The inventors of the present invention have found that, as the contentof Al₂O₃ in a glass phase remaining in a crystallized glass increases,the degree of the coloring thereof can be reduced. For such occasions,it is effective to increase the content of Al₂O₃ in the composition ofglass before crystallization. In particular, the inventors have foundthat, when the content of Al₂O₃ is set as large as 20.5% or more, thedegree of the coloring caused by components such as TiO₂ and Fe₂O₃ andprevailed by SnO₂ can be reduced. However, when the content of Al₂O₃ inthe composition of glass before crystallization exceeds a given content,the excessive Al₂O₃ are mostly distributed into a crystal phase duringthe crystallization, and hence the content of Al₂O₃ in the remainingglass phase is difficult to increase. Thus, it would be insufficient forthe reduction of the coloring to merely increase the content of Al₂O₃ inthe composition of glass before crystallization.

Then, various studies have been made into other components. As a result,the inventors have found that, when the value of Li₂O+0.741MgO+0.367ZnOis set as small as 4.5 or less, the content of Al₂O₃ in the remainingglass phase in the crystallized glass is likely to increase, thus beingable to reduce the degree of the coloring. This can be explained asfollows. Li₂O, MgO, and ZnO tend to precipitate in the crystal phasetogether with Al₂O₃. When the content of these components are decreased,the amount of Al₂O₃ distributed in the crystal phase can be reduced anda larger amount of Al₂O₃ can be distributed in the glass phase. Notethat the coefficients of MgO and ZnO are added for calculating thecontent of each component in terms of Li₂O mole.

The inventors have additionally found that SrO and CaO are also involvedin the coloring as other influential factors. When the value ofSrO+1.847CaO is set as small as 0.5 or less in addition to theabove-mentioned limitation of the composition, a crystallized glasshaving a smaller degree of coloring can be provided. Note that thecoefficient of CaO is added for calculating the content of CaO in termsof SrO mole.

Further, it is also necessary to control strictly the content of each ofTiO₂ and ZrO₂, which are crystal nucleation agents . As describedpreviously, as the content of TiO₂ is larger, the amount of crystalnuclei is likely to increase, and hence the glass hardly becomes cloudy,meanwhile the problem of prevailed coloring occurs. Further, as thecontent of ZrO₂ is larger, the amount of crystal nuclei is likely toincrease, and hence the glass hardly becomes cloudy, meanwhile thedevitrification becomes prevailed, which leads to some problems in theforming step. Thus, while taking the content of Al₂O₃, the value ofLi₂O+0.741MgO+0.367ZnO, and the like into consideration, the inventorshave made studies into the proper ranges of the content of TiO₂ and thecontent of TiO₂+ZrO₂. As a result, the inventors have found that, whenboth the contents are controlled within the above-mentioned ranges, acrystallized glass which has a desired color tone and are less cloudy,thus having high transparency can be obtained.

The Li₂O—Al₂O₃—SiO₂-based crystallized glass of the present invention ispreferable to have a transparent outer appearance.

In this description, the Li₂O—Al₂O₃—SiO₂-based crystallized glass havinga transparent outer appearance is also referred to as“Li₂O—Al₂O₃—SiO₂-based transparent crystallized glass.”

The Li₂O—Al₂O₃—SiO₂-based crystallized glass of the present invention ispreferable to comprise a β-quartz solid solution as a main crystal.

The Li₂O—Al₂O₃—SiO₂-based crystallized glass of the present invention ispreferable to comprise 0.1% or more of MgO.

A thermal expansion property is given as an important property inaddition to the properties regarding outer appearance such as coloringand cloudiness. When the Li₂O—Al₂O₃—SiO₂-based crystallized glass of thepresent invention is used for thermal resistant applications, a thermalexpansion coefficient thereof is preferably as close to zero as possiblein order to reduce a breakage risk. The inventors have made variousstudies also into the relationship between the thermal expansioncoefficient and each component, and have found that, when theLi₂O—Al₂O₃—SiO₂-based crystallized glass comprises 0.1% or more of MgOin the composition range described above, the thermal expansioncoefficient thereof can be made close to zero.

The Li₂O—Al₂O₃—SiO₂-based crystallized glass of the present invention ispreferable to be substantially free of Nd₂O₃ and CoO.

Being substantially free of Nd₂O₃ and CoO each serving as a colorant, acrystallized glass having excellent transparency can be obtained. Notethat the phrase “substantially free of Nd₂O₃ and CoO” means that thesecomponents are not added intentionally, and specifically denotes thatthe content of Nd₂O₃ is 100 ppm or less and the content of CoO is 20 ppmor less.

The Li₂O—Al₂O₃—SiO₂-based crystallized glass of the present invention ispreferable to comprise a restricted content of Fe₂O₃ of 30 to 300 ppm.

Fe₂O₃ is a coloring component that is liable to be contaminated as animpurity, and when the content of Fe₂O₃ is restricted to theabove-mentioned range, the degree of the coloring caused by Fe₂O₃ can bereduced.

The Li₂O—Al₂O₃—SiO₂-based crystallized glass of the present invention ispreferable to have, as a color tone of transmitted light at a thicknessof 3 mm, a b* value of 4.5 or less in terms of L*a*b* representationbased on a CIE standard.

The Li₂O—Al₂O₃—SiO₂-based crystallized glass of the present invention ispreferable to have a transmittance of 82.5% or more at a thickness of1.1 mm and a wavelength of 400 nm.

The Li₂O—Al₂O₃—SiO₂-based crystallized glass of the present invention ispreferable to have a thermal expansion coefficient of −2.5×10⁻⁷/° C. to2.5×10⁻⁷/° C. over 30 to 380° C.

The Li₂O—Al₂O₃—SiO₂-based crystallized glass of the present invention ispreferable to comprise a β-spodumene solid solution as a main crystal.

The Li₂O—Al₂O₃—SiO₂-based crystallized glass of the present inventionpreferably has a white outer appearance.

In this description, the Li₂O—Al₂O₃—SiO₂-based crystallized glass havinga white outer appearance is also referred to as “Li₂O—Al₂O₃—SiO₂-basedwhite crystallized glass.”

The Li₂O—Al₂O₃—SiO₂-based crystallized glass of the present inventionpreferably satisfies the relationship of 0.6≦BaO+2.474Na₂O+1.628K₂≦3.3.

The Li₂O—Al₂O₃—SiO₂-based crystallized glass of the present invention ispreferable to comprises, in terms of mass %, 0.1% or more of each ofBaO, Na₂O, and K₂O.

The Li₂O—Al₂O₃—SiO₂-based crystallized glass of the present invention ispreferable to have a thermal expansion coefficient of 15×10⁻⁷/° C. orless over 30 to 750° C.

The Li₂O—Al₂O₃—SiO₂-based crystallized glass of the present invention ispreferable to have a dielectric loss of 48×10⁻³ or less at a frequencyof 2.45 GHz.

The present invention also presents a method of producing theabove-mentioned Li₂O—Al₂O₃—SiO₂-based crystallized glasses, comprisingthe steps of melting a glass under conditions of a highest temperatureof less than 1780° C. and a melting efficiency of 1 to 6 m²/(t/day),forming the molten glass into a predetermined shape, thereby providing acrystallizable glass, and applying heat treatment to the crystallizableglass, thereby causing crystallization.

The degree of the coloring of the crystallized glass is also influencedby the melting conditions of glass in addition to the composition of theglass . Particularly when SnO₂ is added, the coloring tends to prevailas the molten glass verges on reduction. This is probably because Sn²⁺has a larger degree of influence on the coloring than Sn⁴⁺. In order toprevent the molten glass from verging on reduction to the maximumextent, it is preferred that the melting temperature be lowered and themelting time be shortened. In order to determine the melting time, themelting efficiency (melting area/flow rate) of glass can be adopted asan index of the melting time. Thus, by restricting the meltingtemperature and the melting efficiency within the above-mentionedranges, the molten glass is inhibited from verging on reduction, therebybeing able to provide the crystallized glass in which the degree ofcoloring is reduced.

DESCRIPTION OF EMBODIMENTS

A Li₂O—Al₂O₃—SiO₂-based crystallized glass according to the presentinvention comprises, as a composition in terms of mass %, 55 to 75% ofSiO₂, 20.5 to 27% of Al₂O₃, 2% or more of Li₂O, 1.5 to 3% of TiO₂, 3.8to 5% of TiO₂+ZrO₂, and 0.1 to 0.5% of SnO₂, and satisfies therelationships of 3.7≦Li₂+O+0.741MgO+0.367ZnO≦4.5 and SrO+1.847CaO≦0.5.

Hereinafter, the reasons why the content of each component in theLi₂O—Al₂O₃—SiO₂-based transparent crystallized glass is specified asabove are described below. Note that in the description of the contentrange of the each component, “%” refers to “mass %,” unless otherwisespecified.

SiO₂ is a component that forms the network of glass and constitutes aLi₂O—Al₂O₃—SiO₂-based crystal. The content of SiO₂ is 55 to 75%,preferably 58 to 70%, particularly preferably 60 to 68%. When thecontent of SiO₂ is less than 55%, the thermal expansion coefficienttends to increase, with the result that it becomes hard to obtain acrystallized glass excellent in thermal shock resistance, and moreover,the chemical durability thereof tends to deteriorate. On the other hand,when the content of SiO₂ is more than 75%, the meltability of the glassdeteriorates, the viscosity of the molten glass becomes larger, andhence the glass tends to be hard to be fined or formed.

Al₂O₃ is a component that forms the network of glass and constitutes aLi₂O—Al₂O₃—SiO₂-based crystal. Further, as described previously, whenAl₂O₃ is present in a glass phase remaining in a crystallized glass, thedegree of the coloring caused by TiO₂ and Fe₂O₃ and prevailed by SnO₂can be reduced. The content of Al₂O₃ is 20.5 to 27%, preferably 21 to25%, particularly preferably 21.5 to 23%. When the content of Al₂O₃ isless than 20.5%, the thermal expansion coefficient tends to increase,with the result that it becomes hard to obtain a crystallized glassexcellent in thermal shock resistance, and moreover, the chemicaldurability tends to deteriorate. In addition, it becomes hard to obtainthe effect of reducing the degree of the coloring caused by TiO₂ andFe₂O₃ and prevailed by SnO₂. On the other hand, when the content ofAl₂O₃ is more than 27% in glass, the meltability of the glassdeteriorates, the viscosity of the molten glass becomes larger, andhence the glass tends to be hard to be fined or formed. In addition,mullite crystals are liable to precipitate to devitrify the glass andthe glass is liable to break.

Li₂O is a component that constitutes a Li₂O—Al₂O₃—SiO₂-based crystal,therefore gives a significant influence on the crystallinity, and thatlowers the viscosity of glass, thereby improving the meltability andformability of the glass. The content of Li₂O is 2% or more, preferably2.5% or more. When the content of Li₂O is less than 2%, mullite crystalsare liable precipitate to devitrify the glass. Moreover, when the glassis crystallized, Li₂O—Al₂O₃—SiO₂-based crystals becomes hard toprecipitate, and hence it is hard to obtain a crystallized glassexcellent in thermal shock resistance. In addition, the meltability ofthe glass deteriorates, the viscosity of the molten glass becomeslarger, and hence the glass tends to be hard to be fined or formed. Onthe other hand, when the content of Li₂O is too large, the crystallinityof the glass becomes too strong, with the result that the glass tends todevitrify and the glass is liable to break. Thus, the content of Li₂O ispreferably 4.5% or less, particularly preferably 4% or less.

TiO₂ is a component that serves as a crystal nucleation agent forcausing crystals to precipitate in a crystallization step. The contentof TiO₂ is 1.5 to 3%, preferably 1.6 to 2 .5%, particularly preferably1.7 to 2.30. When the content of TiO₂ is more than 3% in glass, thecoloring of the glass tends to prevail, and also the glass tends todevitrify, causing to be liable to break. On the other hand, when thecontent of TiO₂ is less than 1.5%, crystal nuclei is not formedsufficiently, with the result that coarse crystals precipitate, leadingto clouding or breakage of the resultant crystallized glass.

In the Li₂O—Al₂O₃—SiO₂-based crystallized glass of the presentinvention, the value of Li₂O+0.741MgO+0.367ZnO falls within the range of3.7 to 4.5, preferably 3.8 to 4.4, particularly preferably 3.8 to 4.2.When the value of Li₂O+0.741MgO+0.367ZnO is more than 4.5, the contentof Al₂O₃ in the glass phase in the crystallized glass decreases, andhence it becomes hard to obtain the effect of Al₂O₃ on suppressingcoloring. On the other hand, when the value of Li₂O+0.741MgO+0.367ZnO isless than 3.7, the grain diameter of a Li₂—Al₂O₃—SiO₂-based crystal inthe crystallized glass becomes larger, and the crystallized glass isliable to be cloudy, with the result that the transparency of thecrystallized glass may be impaired.

Note that the content of each of the MgO and ZnO components is notparticularly limited as long as the above-mentioned range is satisfied,but the content is preferably restricted, for example, to the followingrange.

MgO is a component that dissolves in a Li₂O—Al₂O₃—SiO₂-based crystal andhas the effect of increasing the thermal expansion coefficient of theLi₂O—Al₂O₃—SiO₂-based crystal. The content of MgO is preferably 0 to 2%,0.1 to 1.5%, 0.1 to 1.3%, particularly preferably 0.1 to 1.2%. When thecontent of MgO is more than 2% in glass, the crystallinity may becometoo strong, with the result that the glass tends to devitrify and theglass is liable to break.

ZnO is a component that dissolves in a Li₂O—Al₂O₃—SiO₂-based crystal asMgO is. The content of ZnO is preferably 0 to 2%, 0 to 1.5%,particularly preferably 0.1 to 1.2%. When the content of ZnO is morethan 2% in glass, the crystallinity thereof may become too strong, andhence, when the glass is formed while being cooled mildly, the glasstends to devitrify. As a result, the glass is liable to break, and henceit is difficult to form the glass, for example, by a float method.

In the Li₂O—Al₂O₃—SiO₂-based crystallized glass of the presentinvention, the value of SrO+1.847CaO falls within the range of 0.5 orless, preferably 0.4 or less, particularly preferably 0.2 or less. Whenthe value of SrO+1.847CaO is more than 0.5, the degree of the coloringof the crystallized glass increases and the crystallized glass is alsoliable to be cloudy.

Note that the content of each of the SrO and CaO components is notparticularly limited as long as the above-mentioned range is satisfied.For example, the content of SrO is limited to preferably 0.5% or less,particularly preferably 0.3% or less, and the content of CaO is limitedto preferably 0.2% or less, particularly preferably 0.1% or less.

SnO₂ is a component that serves as a fining agent. The content of SnO₂is 0.1 to 0.5%, preferably 0.1 to 0.4%, particularly preferably 0.1 to0.3%. When the content of SnO₂ is less than 0.1%, it becomes hard toobtain the effect of SnO₂ as a fining agent. On the other hand, when thecontent of SnO₂ is more than 0.5%, the coloring caused by TiO₂ and Fe₂O₃becomes prevailed excessively, and hence the crystallized glass isliable to be yellow-tinged. In addition, the glass is liable todevitrify.

The crystallized glass is preferable to be substantially free of Nd₂O₃and CoO serving as colorants, because Nd₂O₃ and CoO reduce thetransparency of the crystallized glass. In particular, CoO has a verystrong coloring ability, and even a trace amount thereof causes asignificant change in the color tone of the crystallized glass. Thus, itis preferred that the Li₂O—Al₂O₃—SiO₂-based crystallized glass of thepresent invention be substantially free of Nd₂O₃ and CoO serving ascolorants. With this, it becomes possible to obtain aLi₂O—Al₂O₃—SiO₂-based transparent crystallized glass having hightransparency and a determinate color tone. Further, Nd₂O₃ is one of therare earths, which leads to increase material cost. Avoiding the use ofNd₂O₃ substantially, an inexpensive Li₂O—Al₂O₃—SiO₂-based crystallizedglass can be obtained. Note that, when priority is put on less coloringrather than higher transparency, Nd₂O₃ may be added at, for example,about 500 ppm.

As for Fe₂O₃ contaminated as an impurity component, the content thereofshould also be limited. The content of Fe₂O₃ is preferably 300 ppm orless, 250 ppm or less, particularly preferably 200 ppm or less. Thecontent of Fe₂O₃ is preferably as small as possible because the degreeof coloring lowers. However, in order to control the content of Fe₂O₃within the range of, for example, less than 60 ppm, it is necessary touse a high purity material or the like, with the result that it becomeshard to provide an inexpensive Li₂O—Al₂O₃—SiO₂-based crystallized glass.

In the Li₂O—Al₂O₃—SiO₂-based transparent crystallized glass of thepresent invention, the following various components may be added inaddition to the above-mentioned components.

ZrO₂ is a crystal nucleation component for causing crystals toprecipitate in the crystallization step as TiO₂ is. The content of ZrO₂is preferably 0 to 3%, 0.1 to 2.5%, particularly preferably 0.5 to 2.3%.When the content of ZrO₂ is more than 3% in glass, the glass tends todevitrify during melting, and hence the glass becomes hard to be formed.

Note that, in the present invention, the content of TiO₂+ZrO₂ is limitedto 3.8 to 5%, preferably 4 to 4.5%. When the content of TiO₂+ZrO₂ fallswithin the above range, it is possible to obtain a crystallized glasswhich has a desirable color tone, is less cloudy, and have hightransparency.

B₂O₃ is a component that promotes the dissolution of a SiO₂ material ina glass melting step. The content of B₂O₃ is preferably 0 to 2%. Whenthe content of B₂O₃ is more than 2%, the thermal resistance of the glasstends to be impaired.

P₂O₅ is a component that promotes the phase separation of glass andassists the formation of a crystal nucleus. The content of P₂O₅ ispreferably 0 to 3%, 0.1 to 3%, particularly preferably 1 to 2%. When thecontent of P₂O₅ is more than 3%, the glass is liable to cause phaseseparation during a melting step, with the result that it becomes hardto obtain the glass having a desired composition and also the glasstends to be opaque.

Further, it is possible to add Na₂O, K₂O, and BaO at a total content ofpreferably 0 to 2%, particularly preferably 0.1 to 2%, in order toreduce the viscosity of the molten glass and improve the meltability andformability thereof. When the total content of these components is morethan 2%, the glass is liable to devitrify.

Note that raw glass materials for the above-mentioned components are asgiven below. Examples of raw glass materials for Li₂O, Al₂O₃, and SiO₂,which are main components, include lithium carbonate, silica sand,silica stone, aluminum oxide, and aluminum hydroxide. Further, spodumenecan be given as an inexpensive Li₂O raw material, but spodumenegenerally includes a high proportion of Fe₂O₃ in many cases, and hencethe usage of spodumene needs to be restricted. As for the othercomponents, raw materials for ZrO₂ often contain Fe₂O₃ as an impurity,and hence it is preferred to use zirconium silicate, in which thecontent of Fe₂O₃ is 0.5% or less, or high-purity ZrO₂ materials.

The Li₂O—Al₂O—SiO₂-based crystallized glass of the present invention haspreferably, as a color tone of transmitted light at a thickness of 3 mm,a b* value of 4.5 or less, particularly preferably 4 or less, in termsof L*a*b* representation based on the CIE standard. Further, theLi₂O—Al₂O—SiO₂-based crystallized glass of the present invention haspreferably a transmittance of 82.5% or more, particularly preferably 83%or more, at a thickness of 1.1 mm and a wavelength of 400 nm

The Li₂O—Al₂O₃—SiO₂-based crystallized glass of the present invention,which is used for heat resistant applications, has preferably a thermalexpansion coefficient as close to zero as possible. Specifically, thethermal expansion coefficient is preferably −2.5×10⁻⁷/° C. to 2.5×10⁻⁷/°C., particularly preferably −1.5×10⁻⁷/° C. to 1.5×10⁻⁷/° C. over thetemperature range of 30 to 380° C. When Li₂O—Al₂O₃—SiO₂-basedcrystallized glass have a thermal expansion coefficient out of theabove-mentioned range, the crystallized glass is more liable to break.

The Li₂O—Al₂O₃—SiO₂-based crystallized glass of the present inventioncan be manufactured by, for example, a production method comprising thesteps of melting a glass batch under the conditions of a highesttemperature of less than 1780° C. and a melting efficiency of 1 to 6m²/(t/day), forming the molten glass into a predetermined shape, therebyproviding a crystallizable glass, and applying heat treatment to thecrystallizable glass, thereby causing crystallization.

The highest temperature during melting step is preferably less than1780° C., 1750° C. or less, particularly preferably 1700° C. or less.When the highest temperature in melting step is 1780° C. or more, a Sncomponent is liable to be reduced and the degree of coloring tends to beprevailed. The lower limit of the highest temperature during meltingstep is not particularly limited, but is preferably 1600° C. or more,particularly preferably 1650° C. or more, in order to causevitrification reaction to progress sufficiently, thereby obtaininghomogeneous glass.

The melting efficiency of glass is preferably 1 to 6 m²/(t/day),particularly preferably 1.5 to 5 m²/(t/day). When the melting efficiencyof glass is less than 1 m²/(t/day), the melting time is too shortened,which leads to too reduced fining time, and hence it becomes hard toobtain a glass excellent in bubble-less quality. On the other hand, whenthe melting efficiency of glass is more than 6 m²/(t/day), a Sncomponent is liable to be reduced and the degree of coloring tends to beprevailed.

A crystallizable glass can be obtained by forming molten glass into apredetermined shape. For such occasions, various forming methods such asa float method, a press method, and a rollout method can be employeddepending on the intended shape of the crystallizable glass.

The crystallized glass can be manufactured from the crystallizable glassas described above, in the following manner. Note that, in the presentinvention, it is also possible to produce both a transparentcrystallized glass and a white crystallized glass each having desiredproperties from one kind of crystallizable glass, by appropriatelychanging the heat treatment temperature (in particular, the heattreatment temperature at the stage of crystal growth). In this case,steps such as a material preparation step, a melting step, and a formingstep, before a crystallization step can be unified, and hence theproduction cost can be suppressed.

The formed Li₂O—Al₂O₃—SiO₂-based crystallizable glass is subjected toheat treatment at 600 to 800° C. for 1 to 5 hours to form crystal nuclei(a crystal nucleation stage), followed by additional heat treatment at800 to 950° C. for 0.5 to 3 hours, thereby causing Li₂O—Al₂O₃—SiO₂-basedcrystals to precipitate as main crystals (a crystal growth stage). Thus,the Li₂O—Al₂O₃—SiO₂-based transparent crystallized glass can beproduced.

Note that, when heat treatment is carried out at a high temperature of1000° C. or more, in particular, 1100° C. or more in the crystal growthstage, it is possible to obtain the white Li₂O—Al₂O₃—SiO₂-basedcrystallized glass in which crystals of a β-spodumene solid solution areprecipitated as main crystals. However, when the heat treatmenttemperature in the crystal growth stage is too high, the growth rate ofa crystal becomes fast, and hence coarse crystals are liable to begenerated. Thus, the upper limit of the heat treatment temperature ispreferably 1150° C. or less, particularly preferably 1145° C. or less.Note that the time period of the heat treatment in the crystal growthstage is appropriately selected, for example, from 0.1 to 3 hours, sothat crystals sufficiently grow and also generation of coarse crystalsis prevented.

Further, as the precipitated crystals are larger, the dielectric lossbecomes higher. Thus, when the crystallized glass is used forapplications in which electromagnetic waves are used, such as a tray fora microwave oven, the temperature thereof locally rises, causing thebreakage thereof . In order to reduce the diameter of crystal grains, itis preferred to set heat treatment conditions so that many nuclei areformed in the crystal nucleation stage. Specifically, the heat treatmentin the crystal nucleation stage is preferably carried out at 700 to 820°C. When the heat treatment is carried out at a temperature lower thanthe range, crystal nuclei become hard to be generated. When the heattreatment is carried out at a temperature higher than the range, crystalgrowth might set in. A time period for crystal nucleation is notparticularly limited as long as sufficient amounts of crystal nuclei aregenerated, and is appropriately selected, for example, from 1 to 5hours.

The Li₂O—Al₂O₃—SiO₂-based white crystallized glass preferably has thesame composition as that of the Li₂O—Al₂O₃—SiO₂-based transparentcrystallized glass described previously, unless otherwise specified.

As the content of alkali or alkaline-earth component is larger in theLi₂O—Al₂O₃—SiO₂-based white crystallized glass, the thermal expansioncoefficient and dielectric loss thereof are liable to increase. This isprobably because alkali or alkaline-earth component generally increasenon-bridging oxygen in glass, and hence, when these components areadded, molecular vibration by thermal energy is enhanced in glass andions become more mobile in glass. Thus, when the content of alkali oralkaline-earth component is decreased, the thermal expansion coefficientand the dielectric loss can be reduced. Note that, even when the contentof alkali or alkaline-earth component is too small, the thermalexpansion coefficient and the dielectric loss are liable to increase,although the detailed mechanism thereof is unclear.

Moreover, alkali or alkaline-earth component serves as meltingaccelerate component, and hence the addition of these componentsprovides the effect of suppressing air bubbles remaining in the glass.Particularly, in the case of avoiding the usage of As₂O₃ or Sb₂O₃ as afining agent, even when SnO₂ is added as a substitute fining agent, airbubbles in the glass are liable to increase. Therefore, the effect ofthe addition of alkali or alkaline-earth component is significant.

In view of the foregoing, in the Li₂O—Al₂O₃—SiO₂-based whitecrystallized glass, it is preferred to adjust the content of alkali oralkaline-earth component, particularly BaO, Na₂O, and K₂O which areliable to influence the above-mentioned effect. Specifically, the valueof BaO+2.474Na₂O+1.628K₂O is preferably 0.6 to 3.3, particularlypreferably 1 to 3.2. Here, the coefficients of Na₂O and K₂O are addedfor calculating the content of each component in terms of BaO mole.

Note that it has been found that the mobility of ions in glass can befurther reduced by the mixed alkali effect. Thus, when glass compriseseach of BaO, Na₂O, and K₂O components at 0.1% or more, the effect ofsuppressing the movement of ions in the glass can be obtained, and hencea lower thermal expansion coefficient and a lower dielectric loss can beachieved.

The Li₂O—Al₂O₃—SiO₂-based white crystallized glass has a thermalexpansion coefficient of preferably 15×10⁻⁷/° C. or less, particularlypreferably 14×10⁻⁷/° C. or less over the temperature range of 30 to 750°C. When the Li₂O—Al₂O₃—SiO₂-based white crystallized glass has a thermalexpansion coefficient exceeding the range, the crystallized glass isliable to break, in a heat resistant application. Note that the lowerlimit of the thermal expansion coefficient is not particularly limited,but is realistically 5×10⁻⁷/° C. or more, particularly realistically10×10⁻⁷/° C. or more.

The Li₂O—Al₂O₃—SiO₂-based white crystallized glass has a dielectric lossof preferably 48×10⁻³ or less, particularly preferably 47×10⁻³ or less,at a frequency of 2.45 GHz. When the Li₂O—Al₂O₃—SiO₂-based whitecrystallized glass has a dielectric loss exceeding the range, thecrystallized glass is liable to break due to a local temperature risethereof, in an application using an electromagnetic wave, such as a trayfor a microwave oven. Note that the lower limit of the dielectric lossis not particularly limited, but is realistically 20×10⁻³ or more,particularly realistically 30×10⁻³ or more, at a frequency of 2.45 GHz.

The Li₂O—Al₂O₃—SiO₂-based crystallized glass of the present inventionmay be subjected to post-processing such as cutting, polishing, orbending processing or to painting or the like on the surface.

EXAMPLES

Hereinafter, the present invention is described in detail by way ofexamples. However, the present invention is not limited to the followingexamples.

Examples 1 to 7 and Comparative Examples 1 to 6

First, raw materials in the forms of an oxide, a hydroxide, a carbonate,a nitrate, and the like were blended and uniformly mixed to obtain aglass having each of the compositions shown in Table 1. The resultantraw material batch was loaded into a refractory furnace of oxygencombustion system, and was then melted under the conditions of a meltingefficiency of 2.5 m²/(t/day) and a highest temperature of 1680° C. Themolten glass was stirred with a platinum stirrer and was then subjectedto roll forming so as to have a thickness of 4 mm, followed by coolingto room temperature in an annealing furnace, thereby obtaining acrystallizable glass.

Heat treatment was applied to the crystallizable glass at 760 to 780° C.for 3 hours to conduct crystal nucleation. After that, the resultant wasadditionally subjected to heat treatment at 870° C. to 890° C. for 1hour, causing crystallization. The resultant crystallized glass wasmeasured for its color tone, transmittance, and thermal expansioncoefficient.

A transparent crystallized glass sheet having a thickness of 3 mm, whichhad been subjected to optical polishing of both surfaces thereof, wasmeasured for its transmittance at a wavelength of 380 to 780 nm by usinga spectrophotometer, and the L*a*b* value based on the CIE standard wascalculated from the transmittance. The color tone of transmitted lightwas evaluated based on the L*a*b* value.

A crystallized glass sheet having a thickness of 1.1 mm, which had beensubjected to optical polishing of both surfaces thereof, was measuredfor its transmittance at a wavelength of 400 nm by using aspectrophotometer. In this manner, the transmittance was evaluated.

A glass sample prepared by processing the crystallized glass into a rodhaving a size of 50 mm in length by 5 mm in diameter was measured forits average linear thermal expansion coefficient in the temperaturerange of 30 to 380° C., and the thermal expansion coefficient of theglass was evaluated based on the average linear thermal expansioncoefficient.

TABLE 1 Example 1 2 3 4 5 6 7 Glass S_(i)O₂ 65.75 65.7 65.7 65.7 65.165.3 65.75 composition A1₂O₃ 22.3 22.2 22.2 22.1 21.9 22.2 22.1 (mass %)Li₂O 3.6 3.57 3.57 3.8 2.94 3.57 4.15 Na₂O 0.35 0.35 0.35 0.35 0.35 0.350.5 K₂O 0.3 0.3 0.3 0.3 0.3 0.3 0.3 MgO 0.7 0.7 0.85 0.7 0.85 0.85 CaOSrO BaO 1.2 1.2 1.2 1.2 1.2 1.7 1.5 ZnO 1.61 TiO₂ 2.0 2.0 2.0 2.0 2.02.0 2.0 ZrO₂ 2.2 2.2 2.2 2.2 2.2 2.2 2.2 P₂O₅ 1.4 1.4 1.4 1.4 1.4 1.41.3 SnO₂ 0.2 0.4 0.2 0.2 0.1 0.1 0.2 Fe₂O₃ (ppm) 200 200 200 200 200 200200 Li₂O + 0.741MgO + 0.367ZnO 4.1 4.1 4.2 4.3 4.2 4.2 4.2 SrO +1.847CaO 0.0 0.0 0.0 0.0 0.0 0.0 0.0 b* value 3.3 3.9 3.4 3.4 3.4 3.53.4 Transmittance at 400 nm (%) 85 83 85 85 85 85 85 Thermal expansioncoefficient −0.9 −0.9 −0.2 −1 1.3 1.1 −8.4 (×10⁻⁷/° C.)

TABLE 2 Comparative Example 1 2 3 4 5 6 Glass S_(i)O₂ 65.2 67.8 64.566.3 66.3 64.1 composition A1₂O₃ 22.1 20.2 22.1 22.3 22.3 22.2 (mass %)Li₂O 4.15 3.57 3.57 2.95 3.57 3.57 Na₂O 0.35 0.35 0.35 0.35 0.35 0.35K₂O 0.3 0.3 0.3 0.3 0.3 0.3 MgO 0.85 0.85 0.85 0.85 0.85 0.85 CaO 0.5SrO 0.5 BaO 1.2 1.2 1.2 1.2 1.2 1.7 ZnO TiO₂ 2.0 2.0 3.3 2.0 1.4 2.0ZrO₂ 2.2 2.2 2.2 2.2 2.2 2.2 P₂O₅ 1.4 1.4 1.4 1.4 1.4 1.4 SnO₂ 0.2 0.10.2 0.1 0.1 0.3 Fe₂O₃ (ppm) 200 200 200 200 200 200 Li₂O + 0.741MgO +0.367ZnO 4.8 4.2 4.2 3.6 4.2 4.2 SrO + 1.847CaO 0.0 0.0 0.0 0.0 0.0 1.4b* value 4.7 5.2 5.4 13.8 5.3 4.7 Transmittance at 400 nm (%) 82.5 81 8069 81 82.5 Thermal expansion coefficient −1.1 −1.8 0.6 4.6 −1.5 2.9(×10⁻⁷/° C.)

As evident from Table 1, it is found that the crystallized glasses ofExamples all had a b* value of as small as 3.9 or less and have atransmittance of as high as 83% or more. In contrast, the crystallizedglasses of Comparative Examples had a b* value of as large as 4.6 ormore. Further, the crystallized glasses of Comparative Examples 2, 4,and 5 had a transmittance of as low as 81% or less.

Example 8

A crystallized glass was manufactured in the same manner as in Example1, except that the raw material batch was melted at a melting efficiencyof 2 m²/(t/day) and a highest temperature of 1820° C. The resultantcrystallized glass was measured for its b* value. As a result, it wasfound that the b* value was larger by about 1 than that of thecrystallized glass of Example 1, and hence the coloring of thecrystallized glass of Example 8 was prevailed.

Examples 9 to 14 and Comparative Examples 7 to 10

Tables 3 and 4 show Examples 9 to 14 and Comparative Examples 7 to 10.

TABLE 3 Example 9 10 11 12 13 14 Glass S_(i)O₂ 65.7 65.7 65.4 65.6 65.865.7 composition A1₂O₃ 22.3 22.2 22.1 22.1 22.2 22.3 (mass %) Li₂O 3.64.2 3.6 3.6 3.6 3.6 Na₂O 0.4 0.5 0.4 0.4 0.4 0.4 K₂O 0.3 0.3 0.3 0.3 0.30.3 MgO 0.7 0.7 0.7 0.7 0.7 ZnO 0.5 0.5 BaO 1.2 1.5 1.2 1.2 0.7 1.2 TiO₂2.0 2.0 2.0 2.0 2.0 2.0 ZrO₂ 2.2 2.2 2.2 2.2 2.2 2.2 P₂O₅ 1.4 1.3 1.41.4 1.4 1.4 SnO₂ 0.2 0.1 0.2 0.5 0.2 0.2 Cl Li₂O + 0.741MgO + 0.367ZnO4.1 4.2 4.3 4.1 4.3 4.1 BaO + 2.474Na₂O + 1.628K₂O 2.7 3.2 2.7 2.7 2.22.7 Nucleation temperature (° C.) 790 790 790 790 790 730 Highesttemperature (° C.) 1130 1145 1145 1130 1130 1130 Thermal expansioncoefficient (×10⁻⁷) 14 12 13 14 12 14 Dielectric loss (×10⁻³) 47 43 4738 45 45

TABLE 4 Comparative Example 7 8 9 10 Glass S_(i)O₂ 64.8 65.3 64.9 65.1composition Al₂O₃ 21.8 22.2 22.2 22.1 (mass %) Li₂O 3.6 3.6 3.6 3.6 Na₂O0.4 0.9 0.4 K₂O 0.3 0.3 0.5 0.3 MgO 0.8 0.8 0.8 1.6 ZnO BaO 2.5 1.2 2.21.2 TiO₂ 2.0 2.0 2.0 2.0 ZrO₂ 2.2 2.2 2.2 2.2 P₂O₅ 1.4 1.4 1.4 1.4 SnO₂0.2 0.1 0.2 0.1 Cl Li₂O + 0.741 MgO + 0.367 ZnO 4.2 4.2 4.2 4.8 BaO +2.474 Na₂O + 1.628 K₂O 4.0 3.9 3.0 2.7 Nucleation temperature (° C.) 790790 790 790 Highest temperature (° C.) 1130 1130 1130 1130 Thermalexpansion coefficient (× 10⁻⁷) 16 17 15 17 Dielectric loss(× 10⁻³) 52 4957 35

Each sample was manufactured as follows. First, raw materials in theforms of an oxide, a hydroxide, a carbonate, a nitrate, and the likewere blended and uniformly mixed to obtain a glass having each of thecompositions shown in the tables, thereby preparing a raw materialbatch. The raw material batch was loaded into a platinum crucible, wasmelted at 1600° C. for 18 hours in an electric furnace, and was thenmelted additionally at 1650° C. for 2 hours. Subsequently, the moltenglass, which was poured from the platinum crucible, was subjected toroll forming so as to have a thickness of 5 mm, followed by cooling toroom temperature in an annealing furnace, thereby obtaining aLi₂O—Al₂O₃—SiO₂-based crystallizable glass.

The resultant Li₂O—Al₂O₃—SiO₂-based crystallizable glass was heated at790° C. for 100 minutes to perform crystal nucleation. After that, theresultant was heated at 1130° C. for 30 minutes, causing crystal growth,thereby obtaining an Li₂O—Al₂O₃—SiO₂-based white crystallized glass.

The resultant Li₂O—Al₂O₃—SiO₂-based white crystallized glass wasevaluated for its thermal expansion coefficient and dielectric loss.

The Li₂O—Al₂O₃—SiO₂-based white crystallized glass was processed into arod having a size of 50 mm in length by 5 mm in diameter, the rod wasmeasured for its average linear thermal expansion coefficient in thetemperature range of 30 to 750° C. by using a dilatometer, and thethermal expansion coefficient of the white crystallized glass wasevaluated based on the average linear thermal expansion coefficient.

The dielectric loss was determined by using a cavity resonator (at ameasurement frequency of 2.45 GHz at 25° C.)

As evident from Tables 3 and 4, it was found that theLi₂O—Al₂O₃—SiO₂-based white crystallized glasses of Examples 9 to 14 hada thermal expansion coefficient of as low as 14×10⁻⁷ or less and adielectric loss of as low as 47×10⁻³ or less.

On the other hand, the Li₂O—Al₂O₃—SiO₂-based white crystallized glassesof Comparative Examples 7, 8, and 10 had a thermal expansion coefficientof as large as 16×10⁻⁷ or more. Further, the Li₂O—Al₂O₃—SiO₂-based whitecrystallized glasses of Comparative Examples 7 to 9 had a dielectricloss of as large as 49×10⁻³ or more.

INDUSTRIAL APPLICABILITY

The Li₂O—Al₂O₃—SiO₂-based crystallized glass of the present invention issuitable for a front window for a kerosene stove, a wood stove, or thelike, a substrate for a high-tech product such as a substrate for acolor filter or an image sensor, a setter for baking an electronic part,a tray for a microwave oven, a top plate for induction heating cooking,a window glass for a fire prevention door, or the like.

1. A Li₂O—Al₂O₃—SiO₂-based crystallized glass, comprising, as acomposition in terms of mass %, 55 to 75% of SiO₂, 20.5 to 27% of Al₂O₃,2% or more of Li₂O, 1.5 to 3% of TiO₂, 3.8 to 5% of TiO₂+ZrO₂, and 0.1to 0.5% of SnO₂, and satisfying the relationships of3.7≦Li₂O+0.741MgO+0.367ZnO≦4.5 and SrO+1.847CaO≦0.5.
 2. TheLi₂O—Al₂O₃—SiO₂-based crystallized glass according to claim 1, whereinthe Li₂O—Al₂O₃—SiO₂-based crystallized glass has a transparent outerappearance.
 3. The Li₂O—Al₂O₃—SiO₂-based crystallized glass according toclaim 1, wherein the Li₂O—Al₂O₃—SiO₂-based crystallized glass comprisesa β-quartz solid solution as a main crystal.
 4. TheLi₂O—Al₂O₃—SiO₂-based crystallized glass according to claim 1, whereinthe Li₂O—Al₂O₃—SiO₂-based crystallized glass comprises 0.1% or more ofMgO.
 5. The Li₂O—Al₂O₃—SiO₂-based crystallized glass according to claim1, wherein the Li₂O—Al₂O₃—SiO₂-based crystallized glass is substantiallyfree of Nd₂O₃ and CoO.
 6. The Li₂O—Al₂O₃—SiO₂-based crystallized glassaccording to claim 1, wherein the Li₂O—Al₂O₃—SiO₂-based crystallizedglass further comprises 30 to 300 ppm of Fe₂O₃.
 7. TheLi₂O—Al₂O₃—SiO₂-based crystallized glass according to claim 1, whereinthe Li₂O—Al₂O₃—SiO₂-based crystallized glass has, as a color tone oftransmitted light at a thickness of 3 mm, a b* value of 4.5 or less interms of L*a*b* representation based on the CIE standard.
 8. TheLi₂O—Al₂O₃—SiO₂-based crystallized glass according to claim 1, whereinthe Li₂O—Al₂O₃—SiO₂-based crystallized glass has a transmittance of82.5% or more at a thickness of 1.1 mm and a wavelength of 400 nm. 9.The Li₂O—Al₂O₃—SiO₂-based crystallized glass according to claim 1,wherein the Li₂O—Al₂O₃—SiO₂-based crystallized glass has a thermalexpansion coefficient of −2.5×10⁻⁷/° C. to 2.5×10⁻⁷/° C. over 30 to 380°C.
 10. The Li₂O—Al₂O₃—SiO₂-based crystallized glass according to claim1, wherein the Li₂O—Al₂O₃—SiO₂-based crystallized glass comprises aβ-spodumene solid solution as a main crystal.
 11. TheLi₂O—Al₂O₃—SiO₂-based crystallized glass according to claim 10, whereinthe Li₂O—Al₂O₃—SiO₂-based crystallized glass has a white outerappearance.
 12. The Li₂O—Al₂O₃—SiO₂-based crystallized glass accordingto claim 10, which satisfies the relationship of0.6≦BaO+2.474Na₂O+1.628K₂O≦3.3.
 13. The Li₂O—Al₂O₃—SiO₂-basedcrystallized glass according to claim 10, wherein theLi₂O—Al₂O₃—SiO₂-based crystallized glass comprises, in terms of mass %,0.1% or more of each of BaO, Na₂O, and K₂O.
 14. TheLi₂O—Al₂O₃—SiO₂-based crystallized glass according to claim 10, whereinthe Li₂O—Al₂O₃—SiO₂-based crystallized glass has a thermal expansioncoefficient of 15×10⁻⁷/° C. or less over 30 to 750° C.
 15. TheLi₂O—Al₂O₃—SiO₂-based crystallized glass according to claim 10, whereinthe Li₂O—Al₂O₃—SiO₂-based crystallized glass has a dielectric loss of48×10⁻³ or less at a frequency of 2.45 GHz.
 16. A method of producingthe Li₂O—Al₂O₃—SiO₂-based crystallized glass according to claim 1,comprising the steps of: melting a glass under conditions of a highesttemperature of less than 1780° C. and a melting efficiency of 1 to 6m²/(t/day); forming the molten glass into a predetermined shape, therebyproviding a crystallizable glass; and applying heat treatment to thecrystallizable glass, thereby causing crystallization.